How to "mix" colors on a shirt? (how to print this basically)

[Fluid]

Get some curable reducer and think your plasticol inks back. You can mimic the hand of water based inks using reducers with your plastisols. 

We printed golf towels for every major College and University this way with great success.  With Curable reducer, you can add more than the "10-15%" to thin back the ink. Just make sure you run some curing tests before the actual order to make sure you didn't reduce the inks beyond proper curing. 

Also Note that reducing your inks will not only change the viscosity of your inks it will change to opacity of them as well. 

[Gilligan]

I used some of that on a job a while back and realized I could have use a lot more than what the instructions said.

It would be good fit for this job, thanks for the heads up on the curable part... at what percentage does that become an issue?